Revelation 3:20 “Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.”

Usually we take this verse as invitation for salvation, and while I do not believe it to be a sin to do so, that is not the actual meaning of the verse.
Jesus is speaking here to the church and those individuals who make up the church. Furthermore He is talking directly to a group that is about as backslidden as you can get. He is already inside but has been assigned a room that does not include the throne room of the heart. The person is saved but Jesus has been stuffed into the prayer closet; a door only to be opened in time of need.
The knocking our Lord is doing is to remind us that He is indeed still here. By knocking He is showing that He has the self control to not rudely barge in where He rightfully belongs, although it is our rudeness towards Him in the first place.
Nor does He stand there banging on the door ordering at the top of His lungs to let Him inside. Note how He said, “if any man hear my voice” that shows He will even ask quietly and politely to be let back in. Of course being able to hear Him also speaks of how difficult we have made it for ourselves to hear Him. The more we put between us and Jesus the harder time we will have.
Finally Jesus said if He is allowed to enter He will “sup” with the individual (or church, because many churches have locked Him out). As that Jesus is the bread of life and the water of life the analogy is most fitting. While we would not dining on His actual flesh (John 6:53-64) we do feast on the Word of God or study the Bible. We can try reading the Bible but if we are not prayed up and in His will it is just words that will mean nothing to us. If we are in His will and seeking Him the words in the Bible will virtually jump off the pages at us.
First though we have to get Jesus out of our closet and back in His throne in our heart.